The present invention generally relates to semiconductor plasma etching. More particularly, the invention relates to a radio frequency (RF) probe analysis system having a broadband design.
In the semiconductor industry, plasma etching has become an integral part of the manufacture of semiconductor circuits. In fact, etchers are frequently used in semiconductor processing when a relatively straight vertical edge is needed. For instance, when etching the polysilicon gate of a MOS transistor, undercutting the polysilicon can adversely affect the operation of the transistor. Undercutting is frequently encountered when a liquid etching method is used. As a result, other etching techniques such as plasma etching have evolved. Plasma etching, which uses ions accelerated by an electric field, tends to etch only horizontal exposed surfaces and therefore avoids undercutting.
In order to effectively execute a plasma etching process (as well as any other plasma process), it has been determined that a system for controlling the power delivered to the plasma chamber highly desirable. In fact, as the complexity of etching processes increases, strict requirements to control the power to accuracies as high as plus or minus one percent have evolved. Thus, in addition to the power delivery system, a closed loop control system is typically employed to monitor the amount of power actually being delivered to the chamber.
FIG. 7 therefore shows a conventional configuration wherein a power delivery system 11 supplies a plasma chamber 13 with RF power for the purpose of executing a plasma process such as etching. Generally, a closed loop control system 15 monitors the RF power, and provides various control signals to the power delivery system 11.
Conventional approaches to the control system 15 have been unable to meet the increasingly strict tolerance requirements of modern day plasma processes for a number of reasons. One particular reason is that the RF power typically has multiple fundamental frequencies as well as corresponding harmonic tones. For example, the voltage applied to the chamber 13 may have fundamental frequencies at both 2 MHz and 27 MHz. The same is true for the current. The 2 MHz signal will have harmonics at 4 MHz, 6 MHz, 8 MHz, etc. The 27 MHz signal will have intermodulation products at 25 MHz, 23 MHz, etc. This is significant because knowledge of the amount of energy present at all of these frequencies is needed for tighter control over the power delivered to the chamber 13. For example, a digital spectrum signal representing the amount of instantaneous power at each of these frequencies would enable the power delivery system 11 to adjust various internal parameters on a frequency-specific basis. Other useful signals include digital magnitude and digital phase signals.
The conventional closed loop control system 15, however, has a sampling unit that treats each frequency of interest individually by mixing the particular signal with an intermodulation signal (such as a signal 10 kHz greater than the frequency of the signal of interest). The mixing results in a second order intermodulation product at 10 kHz, and this product is sampled using an audio grade digitizer. Thus, the result is a plurality of digital power signals (generated by a plurality of digitizers), where each digital power signal corresponds to a mixed frequency signal. It is easy to understand that in the above example, wherein a number of fundamental frequencies as well as harmonic frequencies are present, the sampling unit can become quite complex and expensive. Furthermore, the desire to simultaneously track harmonics cannot be fully satisfied under this approach.
Another difficulty associated with the conventional control system 15 relates to the fact that frequency tuning (to improve impedance matching) is becoming more popular in the semiconductor industry. Thus, prior knowledge of even the fundamental frequencies is often difficult to predetermine. Furthermore, it is envisioned that pulsing power to the chamber will become more prevalent, thereby requiring a response time for the sampling circuit that is unachievable under the conventional approach.
It is important to note that after the RF signals have been digitized into the digital power signals, a fast fourier transform (FFT) is typically performed on a complex composite signal formed from the sampled voltage signal and the sampled current signal for each individual frequency. Once again, the processing overhead associated with such an approach is quite high due to the individual treatment of frequencies. Furthermore, the conventional approach typically employs analog circuitry, which inherently can possess filters with non-linear phase response, and lacks resiliency to tolerances aging and drift. With specific regard to channel-to-channel matching, it has been experienced that the conventional design does not provide a system that is resilient to tolerances, aging, drift or calibration.
The above and other objectives are provided by a radio frequency (RF) probe analysis system in accordance with the present invention. The probe analysis system (operating in either a broadband spectral analysis mode or a broadband digital mixing mode) includes a sampling unit for generating digital power signals based on a plurality of analog signals. The analog signals characterize power delivered from a RF power delivery system to a plasma chamber. The probe analysis system further includes a digital processing unit for generating a digital spectrum signal based on the digital power signals. The sampling unit simultaneously samples a first plurality of frequencies (i.e., bandwidth) from the analog signals such that the digital spectrum signal defines signal levels for the first plurality of frequencies. The sampling unit therefore has a broadband architecture and ameliorates the tolerances of conventional approaches for closed loop control of power delivered to plasma chambers.
Further in accordance with the present invention, a sampling unit for an RF probe analysis system is provided. The sampling unit has a first filtering module and a primary analog-to-digital (A/D) converter. The first filtering module band limits an analog voltage signal and an analog current signal to a first predetermined bandwidth. The analog voltage signal and the analog current signal have a first plurality of frequencies, where the first predetermined bandwidth includes the first plurality of frequencies. The primary A/D converter is coupled to the filtering module and generates a first digital voltage signal based on the analog voltage signal. The primary A/D converter also generates a first digital current signal based on the analog current signal. The primary A/D converter has a dual channel capability such that the first digital voltage signal and the first digital current signal are synchronized.
In another aspect of the invention, a method for analyzing RF power delivered to a plasma chamber is provided. A first plurality of frequencies is simultaneously sampled from a plurality of analog signals. The analog signals characterize the RF power delivered to the chamber. The method further provides for generating digital power signals based on the analog signals, and generating a digital mixing signal having a predetermined mixing frequency. The digital power signals are down converted based on the digital mixing signals such that the first plurality of frequencies are reduced in accordance with the predetermined mixing frequency. The method further provides for reducing a data quantity of the digital power signals, and band limiting the digital power signals based on a fundamental frequency of the first plurality of frequencies. In phase (I) and quadrature (Q) signals are generated based on the digital power signals and a desired sampling rate. The method further provides for generating digital magnitude signals and digital phase signals based on the I and Q signals. Digital spectrum signals are then generated based on the digital phase signals. The digital spectrum signals therefore define signal levels for the first plurality of frequencies.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.